The invention relates to a centerless cylindrical grinding machine for grinding workpieces in rod form with a cylindrical outer contour using the throughfeed grinding process having certain features. The invention also relates to a method for the centerless cylindrical grinding of workpieces in rod form having certain features. A cylindrical grinding machine and a method of this type are known from DE 101 00 871 C1. In the cylindrical grinding machine according to the prior art, two individual cylindrical grinding machines are combined as separate assemblies on a common base plate to form one unit. The rod-shaped or tubular workpiece runs continuously through the two separate grinding units for grinding one after another. Here, in every grinding unit, a comparatively wide cylindrical grinding wheel lies opposite an assembly of regulating wheels which are comparatively thin and are arranged at a spacing from one another on a common spindle. Together with the customary support blade, the grinding and regulating wheels are situated in a common axial region; the two assemblies are also independent of one another in every regard. For example, the grinding geometry, that is to say the spatial assignment of grinding wheel, regulating wheel and support blade in relation to the workpiece, can be different in each of the two units.
In the known cylindrical grinding machine, two grinding gaps are therefore formed which are situated at an axial spacing from one another and through which the rod-shaped or tubular workpiece runs. The two units of the known cylindrical grinding machine can serve different tasks; for example, the rough grinding can take place in the first unit, whereas the finishing is performed in the second unit. However, the finishing can also already be started in the first unit, with the result that more machining time is available overall for the process of finishing. As a result, the tool wear can be reduced considerably during rough grinding with a relatively low erosion rate. In both units of the known grinding machine, the workpiece is arranged in the grinding gap in such a way that it is situated “below the middle”. The following is therefore meant in an exact definition: the workpiece is fixed in the radial direction in the widening grinding gap in accordance with a reference plane which is placed through the rotational and drive axes of regulating wheel and grinding wheel; here, the workpiece longitudinal axis is situated such that it is moved away from said reference plane within a part region of the grinding gap between said reference plane and the supporting face of the work rest blade. This arrangement has the advantage that the workpiece is clamped in a certain way in the grinding gap between the regulating and grinding wheel on one side and the supporting face of the work rest blade on the other side. It therefore also cannot be ejected out of the grinding gap when machining is carried out with relatively great grinding forces. The cylindrical grinding can therefore be carried out “below the middle” with a high material removal rate, and the axial advancing of the workpiece in the grinding section and in the grinding gap can be high.
The arrangement “below the middle” is therefore preferred in many applications in centerless cylindrical grinding. The limits of the arrangement are shown, however, when rod-shaped or tubular workpieces are to be ground which have a small diameter. The workpiece then has to bear against the grinding and regulating wheel in a region of the grinding gap, in which the outer contours of the wheels already merge into an approximately parallel profile. The workpiece therefore lies very far to the top in the grinding gap, with the result that it can migrate upward out of the grinding gap in the worst case. It at least becomes more and more difficult to ensure a secure and steady position of the workpiece during grinding by way of the usual support blade. If the grinding gap becomes narrower and narrower, finally the region is approached, in which the grinding wheels would abut one another; centerless cylindrical grinding of workpieces with a small diameter is then no longer possible in the conventional way.
German patent specification 801 500 has disclosed a special device which is to be actuated by hand and by way of which two lateral grinding points are to be machined on a workpiece at the same time by centerless cylindrical grinding. To this end, two grinding wheels are arranged in a floating manner on a common shaft and are driven rotationally; their axial spacing from one another is variable. In order to grind the lateral grinding points, one movably arranged grinding wheel is moved in axially from the outside toward the second stationary grinding wheel. During the grinding of the lateral grinding points, the two grinding wheels remain at an axial spacing from one another. A regulating wheel for driving the workpiece is arranged opposite on the other side of the workpiece. The regulating wheel and the grinding wheel are offset with respect to one another because, in its axial position, the regulating wheel is situated where the gap between the two grinding wheels exists on the opposite side of the workpiece. In each case only one single workpiece is to be ground by way of the known apparatus, which workpiece has to be introduced into the apparatus and removed from it again for this purpose. The workpiece has approximately the shape of a spindle, as is present in the hubs of bicycles.
In a centerless cylindrical grinding machine which is known from DE 478 720 A, long, thin round rods are to be ground and transported using the throughfeed grinding process by three separate groups of wheels. Each of the three groups comprises a common spindle which is driven rotationally and on which the wheels of said group are situated at a mutual axial spacing. The three groups extend along the round rod to be ground and enclose the latter between them. Here, the first group consists of grinding wheels; the associated spindle extends parallel to the axis of the round rod. The spindle of the second group is inclined slightly with respect to the axis of the round rod; the wheels which are arranged on said spindle are guide wheels with a conical edge which is fitted with felt. The guide wheels bring about the transport of the round rod in the axial direction. The grinding wheels and guide wheels lie opposite one another at a radial spacing in the customary way, to be precise such that exactly one guide wheel lies opposite each grinding wheel.
The third group of wheels in the cylindrical grinding machine according to DE 478 720 A is arranged below the round rod which is running through, on a common spindle in such a way that said wheels engage from below into the axial intermediate spaces between the grinding wheels and the guide wheels and support the round rod. The known machine has no support blade. Rather, the wheels of the third group act in a similar manner to a support blade; in addition, they bring about the rotary drive of the round rod. The known machine according to DE 478 720 A should make the utilization of different-grained grinding wheels possible and at the same time ensure vigorous spinning of the workpieces with powerful axial advancing.